1. Field of the Invention
The present invention relates generally to an actively controlled suspension system having variable of suspension characteristics adapted to vehicular driving conditions for achieving both riding comfort and driving stability. More specifically, the invention relates to an actively controlled suspension system which has variable suspension characteristics depending upon the vehicular driving condition. Further particularly, the invention relates to an actively controlled suspension system which has variable of damping characteristics depending upon the vehicular driving speed for providing satisfactory riding comfort and high driving stability as required depending upon the vehicular driving condition.
2. Description of the Background Art
The U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987, to Hirotsugu YAMAGUCHI et al., and assigned to the common assignee to the present invention, discloses an actively or positively controlled automotive suspension system which includes a hydraulic cylinder for generating a variable damping force against relative motion of a vehicular suspension member and a vehicle body in bounding and rebounding directions for adjusting suspension characteristics and adjusting vehicle body attitude.
Similar type actively controlled suspension systems has been disclosed in the co-pending U.S. patent application Ser. No. 060,911, filed on Jun. 12, 1987 and assigned to the common assignee to the present invention. Further, European Patent First Publications No. 02 49 246 and 02 49 209 respectively corresponding to co-pending U.S. patent application Ser. No. 061,368 (filed on June 15, 1987) and U.S. patent application Ser. No. 059,888, filed on June 9, 1987, both of which have been assigned to the common assignee to present invention. The later-mentioned three inventions includes features of anti-rolling and/or anti-pitching suspension control for adjusting damping characteristics at respective vehicular wheel for suppressing vehicular rolling motion.
Furthermore, the co-pending U.S. patent application Ser. No. 169,201, filed on Mar. 16, 1988, corresponding to the European patent application published under European Patent First Publication No. 0 283 044, co-pending U.S. patent application Ser. No. 176,246, filed on Mar. 31, 1988, corresponding to the European patent application published under European Patent First Publication No. 0 285 153 and the co-pending U.S. patent application Ser. No. 172,419, filed on Mar. 24, 1988, corresponding to the European patent application published under European Patent First Publication No. 0 284 053 have been assigned to the common assignee of the present invention. These co-pending applications disclose actively controlled suspension systems which provide variable suspension characteristics with variable sensibility for vehicular body attitude change.
Such suspension control systems will provide wider flexibility in controlling suspension characteristics and can fit the stiffness level for the drivers' taste. In this sense, the afore-mentioned three inventions are successful in achieving the variable suspension characteristics depending upon vehicle driving condition. However, it would be more desirable to incorporate automatic suspension characteristics adjustment in certain aspect for assuring not only driving stability but only riding comfort.
On the other hand, the conventional active suspension control has been discussed in Proceeding Institution of Mechanical Engineering (Britain) Vol. 185, page 558. Such active suspension control, as discussed, controls vibration transmission ratio between the suspension member and the vehicle body on the basis of a bounding and rebounding acceleration. In the disclosed control, the pressure control valve is controlled for varying fluid path area for changing pressurized fluid supply rate and drain rate for generating damping force against the input shock. For example, assuming input vibration force X.sub.1 is exerted on the suspension member. The piston in the hydraulic cylinder is driving in bounding and/or rebounding direction to cause change of fluid pressure in the hydraulic cylinder. According to this, the working fluid in the fluid chamber communicated with the pressure control valves varies at a speed determined by fluid path area in the pressure control valve. Since the pressure control valve has a throttle for limiting working fluid flow rate, the damping force X.sub.2 is generated in the hydraulic cylinder. Therefore, the vibration transmission ratio becomes X.sub.2 /X.sub.1.
However, in such conventional actively controlled suspension system, the flow-control type servo valve has been employed for controlling the pressure in the hydraulic cylinder. Since such flow-control type servo valve controls working fluid flow rate instead of the working fluid pressure per se, accurate suspension control cannot be achieved unless the vibration transmission ratio is determined in complex manner in relation to the fluid flow rate variation and the corresponding pressure change. This requires substantial capacity and high process speed for the controller to satisfactorily control the suspension in active manner. However, microprocessors for mounting the automotive vehicle has less capacity and process speed than that required for performing active suspension control.
In addition, as will be appreciated, the vehicle body and the suspension member vibrates in various vibration modes, such as pitching mode, rolling mode, bounding mode and so forth. Therefore, as long as suspension control is performed based on the bounding and rebounding acceleration at single point on the vehicle, suspension control covering all of the vibration modes cannot be accomplished.
In order to improve the aforementioned drawback in the prior art, co-pending U.S. patent application Ser. No. 059,888, filed on Jun. 9, 1987, corresponding to the European patent application published under European Patent First Publication No. 0 249 290, proposes an actively controlled suspension system employing a pressure control valve which is operable in response to a control signal and controls fluid pressure in the fluid chamber of a hydraulic cylinder as a replacement of the conventional hydraulic shock absorber. The pressure control valve is associated with a controller which detects bounding and rebounding acceleration and stroke speed in bounding and rebounding motion for deriving a control signal to operate the pressure control valve to absorb vibration energy to be transmitted between a vehicle body and a suspension member.
The prior proposed suspension control system facilitates flexibility in adjustment of the suspension characteristics so that the suspension characteristics may fit the driver's feeling. However, it would be desirable to provide the capability of automatic adjustment of the suspension characteristics in certain vehicle driving condition.